IGNEOUS

Question 1

Silicic rocks are rich in which minerals?

Answer 1

Feldspar and silica (quartz)
Mica (biotite muscovite)
F’spa (potassium plagioclaise)

Question 2

Mafic rocks are rich in which minerals?

Answer 2

magnesium and iron (ferric)
Plagioclaaise f’spa
augite
Olivene

Question 3

fine grained silicic

Answer 3

Pumice Rhyolite

Question 4

fine grained intermediate

Answer 4

Andersite

Question 5

Fine mafic

Answer 5

Bassalt

Question 6

medium mafic

Answer 6

dolerite

Question 7

coarse silicic

Answer 7

Granite

Question 8

intermediate coarse

Answer 8

Diorite

Question 9

Mafic Coarse

Answer 9

Gabbro

Question 10

ultramafic coarse

Answer 10

Peridotite

Question 11

what are the essential minerals?

Answer 11

are the main ones; so mafic rocks largely
consist of plagioclase and a pyroxene such as
augite
.

Question 12

what are accessory minerals?

Answer 12

Occur in smaller quantities

Question 13

quarts:
Composition:
Hardness:
Density:
Colour:
streak:
Cleavage:
Luster:

Answer 13

SiO2
7
2.7
usually white or colourless
White
none
Vistreous

Question 14

Orthoclase feldspar:
Hardness:
Density:
Colour:
streak:
Cleavage:
Luster:

Answer 14

6–6.5
2.6
white or pink short flat crystals
white
2 good. Has the stair-step cleavage characteristic of feldspars. 
vistreous

Question 15

Plagioclase f'spar
Hardness:
Density:
Colour:
streak:
Cleavage:
Luster:

Answer 15

6–6.5 
average
white or gray 
white
2 good
vistreous

Question 16

Olivene:
Hardness:
Density:
Colour:
streak:
Cleavage:
Luster:

Answer 16

never occurs with quarts
 6.5–7 
high
olive green
colourless
poor
vistreous sugary

Question 17

Pyroxene: Augite
Hardness:
Density:
Colour:
streak:
Cleavage:
Luster:

Answer 17

5–6
above average
greenish black
vitreous
2 poor
vistreous

Question 18

AMPHIBOLE:HORNBLENDE
Hardness:
Colour:
Cleavage:
Crystals:

Answer 18

5–6
black, dark green
2 perfact at 60/ 120
needle like crystals

Question 19

Wich rock is amphible common in?

Answer 19

intermediate
rocks such as diorite, less so in granite or
gabbro

Question 20

In which rocks is muscovite mica most aboundant?

Answer 20

Silicic

Question 21

in which rocks is biotite mica most abundant?

Answer 21

Intermediate

Question 22

what makes rocks eqigranular?

Answer 22

Steady cooling

Question 23

what makes rocks glassy?

Answer 23

Instantaneous cooling

Question 24

what is pophoritic texture and how is it caused?

Answer 24

Mixture of grain sizes caused by mixed cooling history; slow
cooling first, followed by a period of somewhat faster
cooling.

Question 25

define:
Phenocrysts:
Groundmass:

Answer 25

large crystals

finer surrounding crystals

Question 26

what is flow banding?

Answer 26

A streaky texture more or less parallel to bedding.

Diagnostic of lavas.

Question 27

Oceanic crust:
composition?
Thickness?
fate?

Answer 27

• Igneous with thin
  sediments. Basalts and gabbros=mafic composition No metamorphics
• 7-10 km
• Subduction

Question 28

Continental crust:
composition?
Thickness?
fate?

Answer 28

• varied with thick sediments and metamorphics Overall silicic with many granites.
• 35 to 40 km
• grows by accretion

Question 29

what is the structure of an ophiolite?

Answer 29

ultramafic rocks at the base, layered gabbros, basaltic dykes
then pillow lavas and then in places,
thin deep water sediments.

Question 30

What are ophiolites?

Answer 30

slices of oceanic crust that have been
trapped and pushed up instead of
being subducted.

Question 31

what is the nature of magmas at a convergent plate margin?

Answer 31

intermediate magma with abundant

andesites and diorites

Question 32

Explain 2 processes caused by subduction

Answer 32

1) Partial melting of the subducted plate and its sediments occurs.Olivines and pyroxenes don’t melt as quickly so we get a more intermediate magma.
2) Volatile materials (mainly water trapped in sediments and in chemically-altered basalts) are liberated as gases. These gases then migrate upwards through the more silicic material of the over-riding plate. This helps melts this to form a more silicic magma which mixes with the rising magma from the subducted plate.

Question 33

What is ‘magma mixing’?

Answer 33

• rising magma mixes with silica rich sediments and rocks of the lithosphere
• a high temperature low silica magma can start melting the overlying crust turning it in to silicic magma
• if these two magmas mix intermediate magma will form

Question 34

What is a ‘Hotspot’. and what is the most common theory for their formation?

Answer 34

regions of particularly high volcanism not associated with plate boundaries.
they are where rising bodies of hot mantle material (‘Mantle Plumes’) have reached the crust.

Question 35

What kind of magma is present at hotspot eruptions and why?

Answer 35

Basaltic magma, As the material rises towards the base of the
lithosphere, the combination of high temperature and a reduction in pressure allows partial melting of the mantle material.

Question 36

why do hotspot volcanoes produce chains of islands?

Answer 36

mantle plumes are at a fixed point in the mantle, an the core moves over the mantle, changing the location on the crust of the eruption of mantle

Question 37

Why does magma form at continent continent plate boundaries?

Answer 37

Neither plate sub ducts, he high pressures and weight of sediments, which have been
deformed to form fold mountains, combine to force the base of the crust down. As a result, the Moho is at its deepest below the highest fold mountains. thus there is high temperatures due to geothermal gradient.

Question 38

What other factors make magmas more likely to form at continent continent plate collisions? what type of magma does this produce?

Answer 38

water and gas lowers the melting point.

Produces silicic magmas and ‘wet’ granites.